Team 696 did two CIMs and an FP with AM Planetary all meshing with a single spur gear in 2005. It worked just fine. I believe it was 16 tooth on the CIMs and 14 on the AM Planetary (for the FP that year).
On the subject of tooth counts for CIM pinions, if the design allows, I would choose a 14 tooth over a 12 tooth, because it will have better meshing characteristics and less wear. I believe a 12 tooth 20DP is undercut (a 14 may be also, but a 12 even more so) which is not preferred. In either case, it would be wise to run a hardened pinion. I believe the Martin S20XX series gears are harder than the AM, or at least have some sort of surface coating that resists wear. We had problems with the as-received 4140 AM 14 tooth gears wearing, even with lubrication. In theory, for a long service life, the pinion should have no fewer than 17 teeth, and the stage ratio should be no greater than 4:1, but in FRC we often break these rules due to the short operation cycles we see here.
Has anyone tried “backyard” hardening of gears? I’d be worried about scale produced on the surface, that would have to somehow be removed.